The present invention relates to methods for ascertaining a correction value for monitoring a fluid bearing of a machine for machining or measuring a workpiece, in particular a coordinate measuring machine, comprising the step of providing the machine for machining or measuring a workpiece, having a first element and a second element, the first element and the second element being supported or being able to be supported against each another by means of at least one fluid bearing, and having a control device for controlling the machine.
Moreover, the present invention relates to a machine for machining and/or measuring a workpiece, in particular a coordinate measuring machine, having a first element and a second element, the first element and the second element being supported or being able to be supported against each other by means of at least one fluid bearing, having a pressure ascertaining device for ascertaining a quantity representing a pressure in the at least one fluid bearing, and having a control device for controlling the machine.
Methods for monitoring fluid bearings and machines, in particular coordinate measuring machines, having monitored fluid bearings are known, for example from the document WO2010/054767 A1.
Coordinate measuring machines are widespread in the prior art. A coordinate measuring machine is a machine having a measuring head which can be moved relative to an object to be measured in a measuring volume. The measuring head is brought into a defined position relative to a measuring point on the object to be measured. In the case of tactile coordinate measuring machines, the measuring point is touched, for example with a feeler pin arranged on the measuring head. Spatial coordinates of the measuring point can subsequently be determined by using the known position of the measuring head in the measuring volume. If the spatial coordinates of a plurality of defined measuring points are determined on an object to be measured, geometric dimensions or even the spatial shape of the object to be measured can additionally be determined. They are used for the purpose of checking workpieces, for example within the context of quality assurance, or for ascertaining the geometry of a workpiece completely within the context of what is known as “reverse engineering”. Furthermore, numerous further possible applications are conceivable.
In such coordinate measuring machines, various types of sensors can be used in order to acquire the coordinates of a workpiece to be measured. For example, sensors measuring in a tactile manner are known for this purpose, such as are marketed by the applicant, for example, under the product designation “VAST”, “VAST XT” or “VAST XXT”. Here, the surface of the workpiece to be measured is touched by a feeler pin, of which the coordinates in the measuring space are continuously known. Such a feeler pin can also be moved along the surface of a workpiece, so that a multitude of measuring points can be acquired at defined time intervals in such a measuring operation within the context of what is known as a “scanning method”.
Furthermore, it is known to employ optical sensors, which permit acquisition of the coordinates of a workpiece without contact. One example of such an optical sensor is the optical sensor marketed by the applicant under the product designation “ViScan”.
The sensors can then be used in various types of measuring structures. One example of such a measuring structure is the product “O-INSPECT” from the applicant. In a device of this type, use is made of both an optical sensor and a tactile sensor, in order to carry out various test tasks on a machine and ideally with a single clamping of a workpiece to be measured. In this way, many test tasks, for example in medical engineering, plastics engineering, electronics and precision mechanical engineering, can be carried out in a straightforward manner. Of course, beyond this, various other structures are also conceivable.
Classically, the sensor head is connected to a support system or machine frame, which supports and moves the sensor system. In the prior art, various support systems are known, for example gantry systems, upright, horizontal-arm and arm systems, all types of robot systems and, finally, self-contained CT systems in the case of sensor systems operating with x-rays. The support systems can, furthermore, have system components which permit the most flexible positioning possible of the sensor head. One example of this is the rotary swivel joint marketed by the applicant under the designation “RDS”. Furthermore, various adapters can be provided in order to connect the different system components of the support system to one another and to the sensor system.
Although the present invention is preferably used in coordinate measuring machines, it can also be used in machine tools and other machines in which a machine head is to be moved with high accuracy relative to a workpiece or the like.
Both machine tools and coordinate measuring machines have a mobile working head. In a coordinate measuring machine, which will be used as an exemplary basis below, the head is frequently fixed to the lower free end of a vertically arranged quill. The quill is movable, so that the measuring head can be moved vertically in relation to a measuring table. The measuring table is used to hold an object to be measured. The quill is in turn arranged on a crossbeam of a gantry, and it can be moved on the crossbeam in a first horizontal direction by means of a carriage. The gantry can be moved together with the quill in a second horizontal direction, so that the measuring head can overall be moved in three mutually perpendicular spatial directions. Here, the quill, the carriage and the gantry form a machine frame. The maximum movement travels of the measuring head along the three directions of movement determine a measuring volume, within which the spatial coordinates can be determined on an object to be measured.
In a similar way, machine tools can be constructed. These typically have, as working head, a spindle having a tool carrier, which is moved in order to machine a workpiece. In addition to air bearings so-called hydraulic bearings, which use a liquid instead of air as fluid, are known, By means of the liquid, a lubricating film, on which two elements can move relative to each other in a sliding manner, is then formed. Within the context of the present application, the term “fluid bearing” is understood to mean both air bearings, that is to say aerostatic or aerodynamic bearings, and also hydraulic bearings, that is to say hydrostatic or hydrodynamic bearings.
Of course, it is not only the machine frame on the sensor-head side of a machine that can be provided with such bearings. A workpiece holder of a machine can for example have air bearings, for example when a workpiece is arranged on a rotary table as workpiece holder. Such a rotary table is used, for example, in the measuring device “PRISMO® Ultra with RT-AB” from the applicant.
Furthermore, the document DE 36 37 410 A1 discloses a method for determining deviations of a rotary table from an ideal rotary axis on a coordinate measuring machine. According to this method, a specific test body having a multitude of defined measuring points is laid on the rotary table, and the positions of the defined measuring points are then ascertained in various angular positions with the aid of the coordinate measuring machine. From the sets of measuring point coordinates, the runout of the rotary axis and the angular position deviation are then determined by computation. In this way, axial deviations of the rotary table, radial deviations and what are known as tumbling deviations in the movement of the rotary table can be determined.
The document DE 34 19 546 A1 discloses a method for ascertaining the position of the center of gravity of a test body, which can be constructed in a very complicated way and can have many individual parts with different specific weights, so that the position of the center of gravity cannot readily be calculated. According to this method, the test body, which can be a motor vehicle, for example, is arranged on a holding plate, which is placed on defined bearing points on the table of a coordinate measuring machine. Load cells are located at the support points of the plate. By using the positional distribution of the support points and the respectively associated support forces, the position of the center of gravity of the test body can be calculated, the initially undefined relative position of the test body in relation to the holding plate being determined with the coordinate measuring machine.
The document EP 0 866 233 A2 shows a device for functional monitoring and stoppage for air bearings, which uses a pressure measurement between the bearing surface and a base plate. A pressure monitor or pressure sensor is connected to an electrical switch, which is in turn connected to the controller of a machine.
In the case of monitored air bearings, the air pressure in the air bearing is measured directly in the bearing gap or in the volume between pressure regulator and outlet opening. The aim of bearing monitoring is normally monitoring the air bearing gap in order to detect overloading of the air bearing. The air bearing pressure and the width or height of the air bearing gap depend on one another here. Furthermore, by using the bearing pressures, for example, conclusions can be drawn about the workpiece moved by an air-mounted table. To this end, combinations of a plurality of individually monitored air bearings are frequently used, in order for example to calculate the center of gravity or the current tilting moment caused by the workpiece. This is described, for example, in the document DE 100 06 876 C1 or in the document WO 2010/054767 A1 cited at the beginning.
In the ideal case, an air bearing loaded with a specific load slides over a perfectly flat mating surface. In this case, the measured air bearing pressure would supply a constant value, which corresponds to this loading state. In reality, however, all the bearing components involved have fabrication tolerances. Air-mounted rotary tables are equipped with face plates of different weights, which can be changed by the user. If only information about the workpiece is then to be obtained, the offsets of the pressure measurement caused by the face plate weight and the fluctuations caused by the fabrication tolerances of the bearing surfaces are disruptive.
In the case of a sensor measuring the pressure in the air bearing relative to an ambient air pressure, weather-dependent fluctuations of the ambient air pressure can additionally form an interfering quantity, which can possibly be eliminated only by means of additional pressure sensor devices. As a result, however, the outlay for the structure once more increases.